This chapter first presents the rationale of this book which reflects research that is undertaken by an independent research consortium, but in close dialogue with EU decision makers. It thus provides critical policy support to address some of the key energy system challenges. The science policy interface established by this research consortium ensures the relevance of this book to a wide audience, both from within and outside of Europe. This includes the European Union, government and international institutions, energy industries, research institutes, and civil society. We then provide an overview of the structure of this book, focused around key policy themes. We first present the current energy landscape, both in terms of the physical system and policy frameworks. The main part of the book then focuses on Europe's energy transition and the associated challenges. Key topics include energy supply, renewable market designs, flexible system operation, and society and consumer demands. The book concludes with an outlook on challenges ahead and policy recommendations.